A single crystal pulling apparatus of this kind consists mainly of a main chamber and a pull chamber, and in the main chamber is provided a crucible for containing the polycrystal substance (raw material) to be single-crystallized, a heater surrounding the crucible, a thermal insulator surrounding the heater, etc. The crucible is fixedly mounted on top of a coaxial vertical support shaft which is adapted to rotate about its axis. The polycrystal substance, such as silicon, charged in the crucible is melted down by the heater to turn into the polycrystal molten liquid, and in this liquid is dipped a seed crystal fixed at the lower end of a pull means which is a wire, etc. and the desired single crystal grows from the seed crystal as the pull means is rotated and raised at predetermined rates together with the seed crystal.
In such a conventional single crystal pulling apparatus, if the crucible were held and maintained at the original altitude, the melt level in the crucible would gradually shift downwards relative to the crucible wall and the heater with the growth of the single crystal, which results in an instability of the thermal condition surrounding the grown crystal and the melt.
Therefore, the support shaft is adapted to axially displace the crucible such that the displacement of the melt level (solid-liquid interface) downwards is compensated for by continuously lifting the crucible at a predetermined rate by a drive means so that the interface level is stationary relative to the heater during the crystal growth.
Incidentally, the diameter of the as-grown single crystal rod (ingot) varies depending on various conditions, of which the pulling velocity (ascending velocity of the pull means) is especially influential, so that in order to obtain a single crystal rod of a uniform diameter, it is important to continually measure the diameter of the as-grown single crystal rod (at the liquid-solid interface) and control the pulling velocity in response to the result of the measurement in a manner such that the diameter is always maintained at a predetermined value, and throughout this crystal pulling operation it is necessary to control the ascending velocity of the crucible in a predetermined proportion to the crystal pulling velocity.
However, in the conventional single crystal pulling apparatus, the heater is stationary so that when the liquid level shifts downwards relative to the crucible wall and the crucible is lifted, the positional relationship between the heater, the crucible wall and the solid-liquid interface changes, whereby the thermal effects of the heater on the solid-liquid interface changes and this typically results in lowering of the oxygen concentration in the growing single crystal. Thus, in a single crystal ingot grown in a conventional single crystal pulling apparatus, the oxygen concentration distribution is such that the oxygen concentration decreases with the growth axis. In order to compensate for the decreasing tendency of the oxygen concentration, it has been practiced to increase the rotational speed of the crucible with the progress of the growth. However, the increased rotational speed has an effect of impairing the uniformity of the oxygen concentration distribution across the cross section of the single crystal ingot.
The present invention was made in view of the above problems, and it is, therefore, an object of the invention to provide a single crystal pulling apparatus which can manufacture a single crystal ingot having uniform oxygen concentration distribution not only with respect to the growth axis but also across cross sections normal to the growth axis.